1. Field of the Invention
The present invention relates to the alignment of a curved continuous web of plastic interlayer material, particularly a length of differentially stretched plastic interlayer material having a shaded band along one side thereof en route from a continuously operating differential web stretching apparatus to an automatic web cutting apparatus that operates intermittently.
2. Technical Background and Description of Patents of Interest
In the preparation of laminated safety glass for one piece windshields of modern automobiles, a thermoplastic sheet is placed between two curved glass sheets of matching configuration. The two glass sheets are bonded to the opposite sides of the plastic sheet by heat and pressure to form the laminated windshield.
The laminated windshields often have an upper colored portion that reduces glare. This coloration in the thermoplasic interlayer sheet is a colored band which is preferably graduated in intensity with the intensity of coloration decreasing from the top of the windshield so that the line of demarcation between the colored band and the uncolored portion of the interlayer is above the direct line of vision. When the interlayer sheet having a rectilinear colored band is used to make a curved windshield, the line of demarcation between the colored and clear portions appears to droop at the ends. However, it is desirable for esthetic purposes to have the line of demarcation between the colored and uncolored portions of the plastic interlayer appear to be horizontal across the entire length of the windshield from side to side of the automobile.
In the past, continuous lengths of plastic interlayer material with such colored bands have been stretched differentially to produce arcuate side edges and a curved line of demarcation between the colored and uncolored portions. For present automobile windshields, a curved line of constant radius of curvature is acceptable. Different constant radii are suitable for different patterns and means are available to differentially stretch plastic interlayer material to a constant radius of curvature for the line of demarcation. The continuous lengths have been cut by hand into shorter lengths compatible with the length of the curved windshield in which the shorter length of interlayer is installed.
U.S. Pat. No. 2,668,705 to Rosenthal and U.S. Pat. No. 3,760,699 to Rosenthal et al. disclose commercially available machinery for automatically cutting different preselected lengths from a continuous web of a flexible material such as paper. These patented web cutting apparatus comprise a transversely extending cutting knife disposed immediately beyond a web driving roll. These apparatus are suitable for intermittently cutting a continuous web of flexible plastic interlayer material into rectangular lengths and requires some modification to be suitable for use with the present invention.
U.S Pat. No. 2,937,407 to Richardson discloses continuously differentially stretching a web of interlayer material that has a dyed band on a conical mandrel while still in ribbon form to distort the web, then cutting the distorted web into sheets while it is still distorted. The cutting is performed manually.
U.S. Pat. No. 3,019,475 to Smith discloses an apparatus comprising a cylindrical heating roll, a conical heating roll and a conical cooling roll disposed along a movement path for differentially stretching a web of interlayer material as the web moves continuously through a web stretching apparatus containing said rolls disposed in tandem so as to produce a curved line of demarcation between the colored portion and the uncolored portion of the web of interlayer material. After the web is differentially stretched, it must be cut into lengths. In order to minimize the loss of plastic interlayer material, the individual pieces are cut manually into quadrilateral shape with the longer curved longitudinal edge to the uncolored side and the shorter curved longitudinal edge to the other colored side and transverse ends interconnecting the curved longitudinal edges.
U.S. Pat. No. 3,696,186 to Stark et al. discloses apparatus for expanding a web of interlayer material by differential stretching between a cylindrical roll and a conical roll which is then fed manually by an operator into a cutting machine for cutting the differentially stretched web into suitable lengths.
U.S. Pat. No. 3,885,899 to Gurta et al. discloses an apparatus for differentially stretching a web of interlayer material which includes alignment means operated by an edge portion of the moving web throttling an air moving means to control the transverse position of a platform mounting a conical stretching member to insure that the web of material is stretched uniformly along extended lengths thereof as it passes over the conical stretching member which stretches the moving web differentially.
U.S. Pat. Nos. 3,912,440 and 3,962,760 to Koss et al. disclose apparatus for continuously stretching a plastic interlayer material that incorporate the invention of U.S. Pat. No. 3,885,899 in combination with other inventions. A delicate spring loaded valve element is an important feature of the conical roll alignment device of these patents. It would be beneficial to avoid such delicate equipment which is likely to require maintenance and repair. The last three patents are assigned to the same assignee as the Stark et al. patent and fail to show any cutting equipment that differs from the manually operated cutting equipment of the Stark et al. patent.
It would be desirable to avoid the need for excess labor that is involved in manually cutting a web of differentially stretched interlayer material.
A continuous web of plastic interlayer material, when continuously stretched differentially from side to side and fed into an intermittently operated web cutting device, forms a loop within a curved length that moves along a path of movement of the differentially stretched web between the continuous differential web stretcher and intermittently operated web cutting device. Early attempts made to cut automatically lengths of differentially stretched plastic interlayer material using the automatic cutting machine available resulted in frustrations.
The automatic cutting machine of the prior art pulled a desired length of web material along a straight path into a cutting station located immediately downstream of a roll drive forming a nip and then stopped the pulling for a cutting operation. The continuous differential web stretcher, on the other hand, caused the web to move continuously therebeyond along an arcuate path. Therefore, the portion of the differentially stretched web with arcuately shaped sides of different lengths between the end of the continuous differential stretching apparatus and the automatic cutting apparatus cyclically developed a veer in the curved length transverse to the direction of web movement so that it was difficult to obtain a cut of proper orientation across the width of the web. Also, the web tended to bunch toward one sheet side. Bunching causes the interlayer material to wrinkle, which is detrimental to the optical properties of the resulting laminated window containing the defective interlayer. This and other problems are solved by the present invention as will be described in further detail subsequently.